1. Field of the Invention
The present invention relates to a dot printer capable of producing dot patterns representing characters, symbols and patterns and of printing the dot patterns on a recording medium.
2. Description of the Related Art
A conventional dot-matrix printer is provided with a print head having a plurality of print wires arranged in a single vertical line and a carriage for moving the print head in a printing direction at a predetermined speed. The print wires are driven selectively according to input data specifying basic dot patterns as the print head is moved at a predetermined speed to print characters and patterns in dot matrices of a predetermined size.
The operating speed of the print wires, namely, the response frequency of the print head, of such a dot-matrix printer is adjusted to a predetermined speed so that dots may be printed accurately at a predetermined pitch with respect to a direction along the line while the carriage moves at a normal speed. Therefore, when the carriage moves at a speed higher than the normal speed for high-speed printing, the pitch of the dots is increased and thereby basic dot patterns are expanded in the direction of the dot line.
Accordingly, dot patterns of a dot density less than that of the basic dot patterns are produced for high-speed printing as a result of omitting every other dot of each dot line of successive dots producing thinned-out dot patterns that are printed during high printing speed, such as twice the normal printing speed (FIG. 5). The result is printed patterns having a thinned-out, or less dense, dot matrix but having the same height and width as those of the basic dot matrix. However, the dot pitch of the thinned-out dot matrix printed at the high printing speed is 1/90 in. whereas the dot pitch of the basic dot matrix printed at the normal printing speed is 1/180 in.
Even when a straight line extending along the direction of the dot line is printed at the high printing speed by omitting every other dot, the deterioration in print quality is insignificant because the thickness of the line, i.e., the size perpendicular to the direction of the dot line, corresponds to the width of the dot and the dot has a fixed size. However, print quality of an oblique or vertical line is deteriorated significantly when the oblique line or the vertical line is printed at the high printing speed. In the thinned-out dot matrix the width of the oblique line or the vertical line is reduced by half. Thus, the conventional dot printer has been unable to print with a satisfactory print quality.
Naturally, the deterioration of print quality may be suppressed by using dot patterns prepared specially for high-speed printing. However, to do so requires dot pattern memories, such as font ROMs for storing both the standard dot patterns and the special dot patterns for high-speed printing and other system components associated with the dot pattern memories.